1. Field of the Invention
This invention concerns a method of making xenobiotic compounds more biodegradable. More specifically, it concerns biodegradable azo dyes.
2. General Discussion of the Background
Azo dyes are important synthetic compounds that are widely used in the dyestuff and textile industries. Unfortunately, they are not biodegradable and tend to persist in the environment unless subjected to costly physical-chemical decontamination processes. Compounds such as azo dyes which resist biodegradation are known as xenobiotics. The azo linkages or aromatic sulfo groups often found in these dyes are generally not synthesized by living organisms, which may help explain their recalcitrance to degradation. Detailed knowledge about biodegradation of these compounds in nature is limited.
Biologic waste treatment processes are sometimes more efficient and less expensive than physical-chemical waste treatment procedures, hence it would be desirable to provide a biological process using microorganisms that degrade xenobiotic azo dyes. Unfortunately, efforts to isolate such microorganisms have been largely unsuccessful in producing a commercially suitable process. Azo dye degrading Pseudomonas strains have been isolated from chemostat cultures by Kulla, "Aerobic bacterial degradation of azo dyes", in Microbial Degradation of Xenobiotic and Recalcitrant Compounds, Academic Press, Inc., London, 1981, pages 387-399 (1981). The degradation mechanism described for that Pseudomonas involved an oxygen-insensitive azoreductase which catalyzed the reductive cleavage of the azo group using NAD(P)H as an electron donor. Zimmerman, et al., Eur. J. Biochem., 1982, 129:197-203. Various anaerobic bacteria that degrade azo dyes have also been reported by Wuhrman, et al., Eur. J. Appl. Microbiol-Biotechnol., 1980, 9:325-338 and Meyer, "Biodegradation of synthetic organic colorants", in Microbial Degradation of Xenobiotic and Recalcitrant Compounds, supra. However, under aerobic conditions these dyes have been considered to be essentially non-biodegradable.
More recently, however, Cripps found that the fungus Phanerochaete chrysosporium aerobically degrades polycyclic hydrocarbons containing azo and sulfo groups. Cripps, et al., Appl. Environ. Microbiol., 1990, 56:1114-1118. That paper described several unidentified metabolites of microbially degraded Tropaeolin O, Congo Red and Orange II after incubation with crude ligninase preparations, but the possible mechanism of degradation was not explained. Other investigators have shown that P. chrysosporium can mineralize chloroaniline/lignin conjugates and xenobiotic molecules bound to humic acids. Haider and Martin, Soil Biol. Biochem., 1988, 20:425-249.
In spite of these advances, the degree of microbial degradation of many azo dyes has remained low. Kulla's azo dye degrading Pseudomonas is highly substrate specific, and requires extensive screening procedures to isolate biodegradative strains. The extreme specificity of Kulla's bacterial strains decreases their practical use in industry because industrial effluents contain mixtures of dyes. Kulla, et al., "Biodegradation of xenobiotics; experimental evolution of azo dye-degrading bacteria", in Current Perspectives in Microbial Ecology, (eds. M. J. Klug and C. A. Reddy), American Society for Microbiology, Washington, D.C., pages 663-667. Moreover, the Pseudomonal strains completely and irreversibly lose their biodegradative ability when grown with the specific substrate for ten generations, as disclosed at page 664 of that publication. Finally, sulfonated aromatic groups in the substrate dyes disturbed the microbial degradative pathways and limited the usefulness of these microorganisms in degrading the vast quantities of industrially produced azo dyes.
Accordingly, it is an object of this invention to provide azo dyes which are more completely biodegradable.
Another object of the invention is to provide such dyes which can be degraded more effectively and discarded less expensively than many previous azo dyes.
Yet another object of the invention is to provide such dyes which are less harmful to the environment.
Another object of this invention is to provide azo dyes which can be degraded by microorganisms with less substrate specificity.
Another object is to provide such dyes which are degraded by relatively common and genetically stable microorganisms that better retain their biodegradative capacity through successive generations.
Finally it is an object of the invention to provide an improved method of treating azo dyes in which sulfonated azo compounds can be degraded.
These and other objects of the invention will be understood more clearly by reference to the following detailed description.